In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have led to rapid progress in the field of miniaturization. Typically, these miniaturization techniques involve shortening of the wavelength of the exposure light source. Until recently, ultraviolet radiation such as g-lines and i-lines has been used as the exposure light source, but recently, mass production using KrF excimer lasers (248 nm) has been started, and even ArF excimer lasers (193 nm) are now starting to be introduced. Radiation of even shorter wavelengths such as F2 excimer lasers (157 nm), EUV (extreme ultraviolet radiation), electron beams, X-rays and soft X-rays is also being investigated.
Resists for use with these types of short wavelength light sources require a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity relative to these types of short wavelength light sources. One example of a known resist that satisfies these conditions is a chemically amplified resist, which includes a base resin and an acid generator (hereafter referred to as a PAG) that generates acid upon exposure, and these chemically amplified resists include positive resists in which the alkali solubility of the exposed portions increases, and negative resists in which the alkali solubility of the exposed portions decreases.
Until recently, polyhydroxystyrene (PHS) or derivative resins thereof in which the hydroxyl groups have been protected with acid-dissociable, dissolution-inhibiting groups (PHS-based resins), which exhibit high transparency relative to a KrF excimer laser (248 nm), have been used as the base resin component of chemically amplified resists. However, because PHS-based resins contain aromatic rings such as benzene rings, their transparency is inadequate for light with wavelengths shorter than 248 nm, such as light of 193 nm. Accordingly, chemically amplified resists that use a PHS-based resin as the base resin component suffer from low levels of resolution in processes that, for example, use light of 193 nm.
As a result, resins that contain structural units derived from (meth)acrylate esters within the principal chain (acrylic resins) are now generally used as base resins for resists that use ArF excimer laser lithography, as they offer excellent transparency in the vicinity of 193 nm. Furthermore in the case of positive resists, as disclosed in the Patent Reference 1 listed below, resins that contain structural units derived from tertiary ester compounds of (meth)acrylic acid such as 2-alkyl-2-adamantyl (meth)acrylates as the structural units containing an acid-dissociable, dissolution-inhibiting group are now widely used. These resins are known to exhibit a high dissociation energy for the acid-dissociable, dissolution-inhibiting groups, meaning there are restrictions on the types of acid generators that can be used within resist compositions that use these resins. In other words, unless an acid generator that generates a strong acid is used, such as an onium salt containing a fluorinated alkylsulfonate ion as the anion, the acid-dissociable, dissolution-inhibiting groups do not dissociate satisfactorily, and the composition is unable to function satisfactorily as a resist.
In recent years, resins containing structural units in which the hydrogen atom of (meth)acrylic acid has been substituted with a so-called acetal group such as a 1-alkoxyalkyl group have been attracting considerable attention as resins containing acid-dissociable, dissolution-inhibiting groups for use within ArF excimer laser lithography and the like (see non-patent reference 1).                [Patent Reference 1]        Japanese Patent (Granted) Publication No. 2,881,969        [Non-Patent Reference 1]        J. Photopolym. Sci. Technol. 17 (2004), 483 to 488        